The present invention relates to a process and apparatus for treating sea water to provide purified potable water.
More particularly, the present invention relates to such a process and apparatus for purifying sea water by means of the phenomenon of reverse osmosis.
For all practical purposes, the supply of fresh potable drinking water to offshore marine structures, such as platforms, ships, barges, weather ships, etc., has been achieved in only two ways, i.e. sea water distillation units and storage tanks.
Sea water distillation units however have the inherent disadvantages of requiring high energy consumption, of being subject to scaling and corrosion, of requiring considerable maintenance, and of not being able to operate in heavy or severe weather conditions.
Furthermore, the supply of fresh drinking water in storage tanks requires that the crew of an offshore marine structure be dependent on outside means of supply. Thus, depending on the geographical location of the offshore structure, fresh water must be supplied by barges, helicopters, etc., and thus the quality of the supplied drinking water will be dependent upon the quality of water in the geographical land supply area. Furthermore, during heavy and severe weather it is difficult to bring such fresh water to the marine structure.
Furthermore, it has been known to purify water by the phenomenon of reverse osmosis. The use of reverse osmosis to purify sea water has particularly been employed since the development by Du Pont of an asymmetrical fiber formed of aromatic polyamids which are naturally stable, which are non-biodegradable, and which can be operated in waters having pH values of from 5 to 9. Such fibers are marketed by Du Pont as B-10 fibers and are particularly useful as a semi-permeable membrane through which sea water may be purified by the phenomenon of reverse osmosis.
However, systems for purifying sea water by reverse osmosis have generally required the necessity of providing a complex pretreatment installation for pretreating the sea water before it contacts the semi-permeable membrane of the reverse osmosis purification unit. Such pretreatment installations have conventionally involved the operations of flocculation, clarification, chlorination and dechlorination, and proportional acidification or addition or an anti-scale agent.
Such pretreatment operations are virtually unusable in offshore marine structures, where space is restricted, where the handling of acid and dangerous chemical reagents is extremely hazardous, and where it is difficult to provide the complex maintenance facilities necessary for such pretreatment operations.
In an attempt to resolve the above problems, a process and apparatus as disclosed in French Patent Application No. 76 02471, published on Aug. 19, 1977 as Publication No. 2,338,901, provided a system whereby sea water could be purified on offshore marine structures by the phenomenon of reverse osmosis to obtain purified potable water, and wherein such system did not require the use of a chemical pretreatment. However, the process and apparatus of such French publication still had certain inherent operational disadvantages. More particularly, although the pretreatment of such French publication is a physical pretreatment, as opposed to a chemical pretreatment, the pretreatment obtained thereby is not entirely satisfactory. Further, the process of the French publication does not entirely eliminate the formation of precipitates, particularly sulfates and carbonates, and more specifically calcium carbonate, on the semi-permeable membrane of the reverse osmosis unit. Even further, during shutdown of the system of the French publication, there occurs the danger of the growth of marine microorganisms in the system, and it is also necessary to clean the system of the French publication at fairly regular intervals.